Alkylation system and a process for cooling a vent stream

ABSTRACT

One exemplary embodiment can be a process for cooling a vent stream from a receiver. Generally, the process may include providing a refrigerant including at least one compound contained in the receiver so the refrigerant leaking into the receiver can be compatible with the process.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Division of copending application Ser. No.12/493,662 filed Jun. 29, 2009, the contents of which are herebyincorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention generally relates to an alkylation system, and a processfor cooling a vent stream from a receiver of a depropanizer column inthe system.

DESCRIPTION OF THE RELATED ART

Often a fractionation zone is positioned downstream of an acidalkylation unit to separate the hydrocarbons into various streams andany remaining acid. In the fractionation zone, often one or more columnsare utilized for providing these separate streams. Usually, the firstcolumn in the series receiving the alkylation zone effluent provides anoverhead stream that can include light hydrocarbons and the acid. Often,the acid can be recycled back to the alkylation unit.

In the receiver of the first column, a chiller can be provided to coolvent gas exiting the receiver. Often a light hydrocarbon can be used asa refrigerant to cool the vent gas.

Typically, the vent gas is at a higher pressure than the refrigerantwithin the coils. As a consequence, the acid alkylation catalyst canleak into the refrigerant system. As a result, the acid can corrodeequipment and lines, such as the refrigerant compressor. Replacing thecompressor and other equipment can incur costs and increase hazards.Thus, it would be desirable to eliminate this risk and lower operatingcosts.

SUMMARY OF THE INVENTION

One exemplary embodiment can be a process for cooling a vent stream froma receiver. Generally, the process may include providing a refrigerantincluding at least one compound contained in the receiver so therefrigerant leaking into the receiver can be compatible with theprocess.

Another exemplary embodiment may be an alkylation system. Generally, thealkylation system may include an acid alkylation zone and afractionation zone. Usually, the fractionation zone has a depropanizercolumn, which can include a receiver. The receiver can include a ventcondenser containing one or more cooling coils, which may contain arefrigerant at a pressure greater than the pressure in the receiver.

Yet another embodiment can include a process for cooling a vent streamfrom a receiver of a depropanizer column in an acid alkylation unit.Generally, the process includes providing a refrigerant includingpropane to one or more cooling coils contained in a vent condenser ofthe receiver. Typically, the propane in the one or more cooling coils isat a pressure greater than the receiver.

The embodiments provided herein can provide a refrigerant at a higherpressure than the gases in the receiver. Typically, leaks can happen dueto cracking of the one or more cooling coils. As a result, therefrigerant can leak into the process rather than the vent gases leakinginto the one or more cooling coils. Typically, the refrigerant can beselected from a light hydrocarbon that is compatible with the process,such as propane, so any leak from the refrigerant into the vent systemcan simply be processed along with the other fluids. Particularly, therefrigerant can be compatible with the overall fractionation zone fluidsso as to be merely separated along with the other products.

Definitions

As used herein, the term “stream” can be a stream including varioushydrocarbon molecules, such as straight-chain, branched, or cyclicalkanes, alkenes, alkadienes, and alkynes, and optionally othersubstances, such as gases, e.g., hydrogen, or impurities, such as heavymetals, and sulfur and nitrogen compounds. The stream can also includearomatic and non-aromatic hydrocarbons. Moreover, the hydrocarbonmolecules may be abbreviated C₁, C₂, C₃ . . . Cn where “n” representsthe number of carbon atoms in the one or more hydrocarbon molecules.Additionally, characterizing a stream as, e.g., a “vent stream”, canmean a stream rich in one or more substances representative of thecharacterizing adjective, such as a vent stream being rich in at leastone fluid typically present in a receiver.

As used herein, the term “zone” can refer to an area including one ormore equipment items and/or one or more sub-zones. Equipment items caninclude one or more reactors or reactor vessels, heaters, exchangers,pipes, pumps, compressors, and controllers. Additionally, an equipmentitem, such as a reactor, dryer, or vessel, can further include one ormore zones or sub-zones.

As used herein, the term “rich” can mean an amount of at least generallyabout 30%, preferably about 50%, and optimally about 70%, by mole, of acompound or class of compounds in a stream or an effluent.

As used herein, the term “substantially” can mean an amount of at leastgenerally about 80%, preferably about 90%, and optimally about 99%, bymole, of a compound or class of compounds in a stream or an effluent.

As used herein, the term “vapor” can mean at least one of a gas or adispersion that may include or consist of one or more hydrocarbons.

As used herein, the term “hydrogen fluoride” can include at least one ofa hydrogen fluoride or a hydrofluoric acid. Generally, a hydrofluoricacid is a solution of a hydrogen fluoride in water, where the hydrogenfluoride can disassociate and may form ions of H₃O⁺, H⁺, FHF⁻, and F⁻.In some preferred embodiments, a substantially anhydrous hydrogenfluoride can be utilized.

As depicted, process flow lines in the figures can be referred to aslines, effluents, or streams. Particularly, a line can contain one ormore effluents or streams, and one or more effluents and streams can becontained by a line.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic depiction of an exemplary alkylation system orunit.

DETAILED DESCRIPTION

Referring to FIG. 1, an acid alkylation system or unit 100 can includean acid alkylation zone 140 and a fractionation zone 200. Typically, theacid alkylation zone 140 can be any suitable alkylation zone, typicallyutilizing an acid alkylation catalyst. Usually, the acid alkylationcatalyst can include a hydrogen fluoride. Exemplary alkylation zones aredisclosed in, e.g., U.S. Pat. No. 5,098,668.

The fractionation zone 200 can include one or more distillation columns,such as a depropanizer column 240. Exemplary distillation columns of thefractionation zone 200 are disclosed in, e.g., U.S. Pat. No. 4,348,544.The depropanizer column 240 can provide an overhead stream 260, aside-stream 270, and a bottom stream 280. Usually, the side-stream 270can include or be rich in one or more hydrocarbons, such as C₃-C₅hydrocarbons, more typically, C₄ hydrocarbons. The bottom stream 280 caninclude or be rich in C₄ ⁺ hydrocarbons, typically an alkylate product.The overhead stream 260 can include or be rich in one or more C₄ ⁻hydrocarbons and an alkylation catalyst, typically an acid such ashydrogen fluoride. The overhead stream 260 can be received within areceiver 300 forming a boot 304. The boot 304 can collect an alkylationcatalyst and provide an alkylation catalyst stream 308 that can berecycled to the acid alkylation zone 140.

The receiver 300 can also provide a hydrocarbon product in a stream 262that can be split into a reflux stream 264 sent back to the depropanizercolumn 240 and an overhead product stream 266, typically includingpropane. In addition, the receiver 300 can form a vent condenser or astack 310 that can provide a vent stream 314 including C₂ ⁻ hydrocarbonswith a reduced level of an acid alkylation catalyst. The vent stream 314can be provided to a scrubber 380 to provide a scrubber effluent stream384 that can be sent to any suitable destination, such as fuel gas orthe flare. Typically, the receiver 300 can be at a pressure of no morethan about 1,720 kPa.

In the vent condenser 310, one or more cooling coils 320 can be providedto cool the vent stream 314 prior to exiting the vent condenser 310 topartially recover propane and the acid alkylation catalyst. Typically,the one or more cooling coils 320 can contain at least a portion of therefrigerant, and the refrigerant can be at a pressure greater than thepressure in the receiver 300. The vent stream 314 can be at atemperature of no more than about −20° C. and a pressure of no more thanabout 1,720 kPa. The refrigerant can enter the one or more cooling coils320 and then exit in a line 322.

Typically, the one or more cooling coils 320 can include any suitablerefrigerant, such as an olefin or a paraffin, in a liquid phase.Usually, a suitable olefin can include a C₂-C₄ olefin, and a suitableparaffin can include propane. Preferably, the refrigerant can be a drypropane obtained from, e.g., a propane stripper or a product dryer,chilled, and pumped to about 60 to about 140 kPa above the pressure inthe receiver 300. In the event of a coil leak, the high-pressure coldpropane as the working fluid in the vent condenser 310 can leak into thedepropanizer column 240, rather than the alkylation catalyst leakinginto the one or more cooling coils 320. Hence, the alkylation catalyst,typically hydrogen fluoride, cannot contaminate the wet gas or fuelsystems. Moreover, a leak can be detected quickly and fixed bymonitoring the refrigerant level in the one or more cooling coils 320 ora surge drum 330.

The line 322 can communicate with the surge drum 330. The refrigerantcan pass through a line 324 to a pump 340, which can pump therefrigerant up to a pressure of about 1,900 kPa.

Afterwards, the refrigerant can pass through a line 326 to the exchanger360. Chilling can be provided by vaporizing the same or differentrefrigerant. Typically, a liquefied petroleum gas stream 364 at apressure of about 300 to about 500 kPa and a temperature of about −5° toabout 5° C. can be provided to cool the vaporized refrigerant. Afterexiting the exchanger 360, the refrigerant can be at a temperature of nomore than about 20° C., preferably below about 0° C. The refrigerant mayreturn to the vent condenser 310 via a line 328.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The preceding preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

In the foregoing, all temperatures are set forth in degrees Celsius and,all parts and percentages are by weight, unless otherwise indicated.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. An alkylation system, comprising: A) an acid alkylation zone; and B)a fractionation zone, comprising a depropanizer column, wherein thedepropanizer column comprises a receiver, wherein the receiver, in turn,comprises a vent condenser containing one or more cooling coils, inturn, containing a refrigerant wherein the refrigerant is at a pressuregreater than the pressure in the receiver.
 2. The alkylation systemaccording to claim 1, wherein the receiver contains an alkylationcatalyst comprising a hydrogen fluoride.
 3. The alkylation systemaccording to claim 1, wherein the refrigerant comprises a propane. 4.The alkylation system according to claim 1, wherein the depropanizercolumn further comprises a pump receiving the refrigerant from the oneor more cooling coils.
 5. The alkylation system according to claim 4,wherein the depropanizer column further comprises an exchanger receivinga refrigerant.
 6. A process for cooling a vent stream from a receiver ofa depropanizer column in an acid alkylation unit, comprising providing arefrigerant comprising a propane to one or more cooling coils containedin a vent condenser of the receiver wherein the propane in the one ormore cooling coils is at a pressure greater than the receiver.
 7. Theprocess according to claim 6, wherein the refrigerant in the one or morecooling coils is at a temperature of no more than about −30° C.
 8. Theprocess according to claim 6, wherein the receiver is at a pressure ofno more than about 1,720 kPa.
 9. The process according to claim 6,further comprising sending the refrigerant contained in the one or morecooling coils to a pump.